1. Field of the Invention
The present invention is related to a registration control device for use in a rotary printing machine.
2. Description of the Prior Art
Registration control devices used in a rotary printing machine usually employ a head for scanning the registration marks, usually printed in the margin, in order to detect any possible misregistration of the prints in each color. The device then generates correction commands which alter either the track of the paper web or the position of the corresponding plate cylinder.
Known devices of this type, such as described in U.S. Pat. No. 3,653,322, have a scanning head which includes one or several photodiodes for scanning a registration mark which enters an illuminated area and travels under the head. In order to compensate unpredictable sideways misregistration inherent to the high running speed of the paper web, the registration marks have in this context the shape of crossed lines with a width considerably broader than the scanning diameter of the photoelectric cell. For instance, with a cell that has a scanning diameter of 6 mm, the width of the mark will perhaps be of 10 mm in order to leave a margin of 2 mm on either side.
In another such device, described in European Application 0 123 305, the registration marks are triangular and tiny, i.e., about 3 mm large by 6 mm long, in order to save room for the prints. The triangular configuration of these registration marks allows the simultaneous determination of the lengthwise (machine-direction) and sideways (cross-machine direction) misregistration of the corresponding print by means of an analysis based on the emission and intensity of the impulse received. However, despite the reduced scanning surface of the head, i.e., of one millimeter or so, it appears necessary to motorize the sideways positioning of the head in order to be able to compensate for a temporary displacement of the web. This motorization of the positioning of the head results in a heavier structure which has repercussions on the realization cost.
As will be understood, the continuous aim for gaining space on the paper, which aim is achieved by reducing the size of the registration marks to approximately 1 mm in width or less, will create a problem in the sideways misregistration of webs, if such misregistration has an amplitude bigger than the one of the registration marks. Motorized correction becomes practically impossible with such small-sized registration marks, which tend to leave the scanning field of the photoelectric cell too easily, even before the determination of their dimensions. Moreover, it is impossible to give such a little mark a particular shape, i.e., that of a triangle, which would allow a simultaneous detection of lengthwise and sideways misregistration.
PCT applications WO 86/05141 and WO 89/01867 suggest solutions employing video cameras taking a global image of a group of marks in order to determine the misregistration of the various colors by means of a numerical analysis based on this image. Although working satisfactorily with prints running at low speed, these devices reach their limit at higher speed, especially when applied to heliographic printing for which the web paper running speed might amount to 20 m/s. At that speed, the image processing should be effectuated at least ten times quicker than is currently done.